Page 38 - From Smart Grid to Internet of Energy

Introduction to smart grid and internet of energy systems Chapter 1 27

improved either with DC sources or with AC sources. Moreover, hybrid micro-
grids including DC and AC DERs are recently being improved. The microgrid
integration to existing distribution system has transformed it from passive struc-
ture to an active distribution system.
Besides its DER types, the microgrids are classified into several groups
regarding to their characteristics, features, operation and control modes. The
operation characteristics and feeder architectures have pave the way to define
microgrids as urban, rural, and off-grid microgrids [19]. The urban microgrids
are installed in concentrated industrial and residential areas as its name implies.
The feeders are in densely populated areas and thus, the distribution lines are
relatively short. One of the main indicators of an urban microgrid is its short
circuit ratio that is determined by the ratio of utility grid short circuit capacity
at the point of common coupling (PCC) to the total DER generation capacity of
the microgrid. Therefore, the short circuit ratio of an urban microgrid should be
over than 25 to manage voltage and frequency by the utility grid during grid-tied
operation. Besides, the transient disturbances are easily tackled in a such inte-
gration. The rural microgrid architectures are widely seen in rarely populated
areas and thus the distribution lines are relatively longer than urban microgrids.
On the other hand, short circuit ratio is not strictly considered but it causes to
several voltage and frequency fluctuations along the microgrid. The microgrid
is much more vulnerable to reliability and stability of DERs in rural architec-
ture. The last prominent architecture is based on off-grid structure that has no
connection to the utility grid and is utilized to supply local loads in geograph-
ically spanned areas [19].
The microgrid integration of RESs and DERs should comply with IEEE
P1547-2003 which is a standard for interconnecting distributed resources with
electric power systems. IEEE P1547 standard series provide several scope and
purpose to microgrid control and management operations. A microgrid central
controller (MGCC) ensures interconnection and coordination of DERs in a
microgrid to utility grid. A MGCC mainly controls the load demand and gen-
eration capacity of plants to manage energy balance between generation and
demand. The MGCC is capable to load shedding to sustain critical load supply
during excessive load demands where the generation has been limited. There-
fore, the non-critical loads are shaded when the demand excesses generation.
Block diagram of microgrid architecture with DERs, MGCC and distributed
controllers have been shown in Fig. 1.10. There are different types of loads
and resources exist in a microgrid and almost all of them are integrated with
utility grid. Therefore, a robust and reliable controller infrastructure is required
to meet operational requirements of microgrid. The MGCC accomplish this task
by monitoring the electrical parameters and making decisions to synchronize, to
connect to grid or disconnect from grid since it is located between utility grid
and microgrid. Once the MGCC decides to disconnect microgrid from utility
grid, then it monitors resynchronization periods to detect restoration of utility
grid and connects the microgrid to restored utility grid in a properly